1. Field of the Invention
The present invention relates to an image display device. More specifically, it relates to an image display device for repeatedly displaying images by driving colored particles sealed between a transparent display substrate and a rear substrate by an electric field.
2. Description of the Related Art
Conventionally, as an image displaying medium (display device) capable of being rewritten repeatedly, a twisting ball display (two-color-painted particle rotating display), an electrophoresis display medium, a magnetophoresis display medium, a thermal rewritable display medium, a liquid crystal display medium having a memory property, and the like have been proposed.
Among these image display media, although the thermal rewritable display medium, the liquid crystal display medium having a memory property, and the like have excellent image memory property, they cannot provide a background with sufficient whiteness like paper, and thus the contrast between an image part and a non-image part is small when displaying an image, so that it has been difficult to display a sharp image.
Moreover, the image media utilizing electrophoresis and magnetophoresis have colored particles movable by, for example, an electric field or a magnetic field dispersed in a white liquid. For imaging, the colored particles are adhered on a display surface so as to display the color of the colored particles in the image part, and the colored particles are eliminated from the display surface so as to display the white color of the white liquid in the non-image part. Since the colored particle movement is not generated except by functioning of the electric field or magnetic field, the display memory property can be provided. However, according to these methods, although the white display property of the white liquid is excellent, when displaying the color of the colored particles, since the white liquid enters into gaps among the colored particles, the display density is lowered. Therefore, the contrast between the display part and the non-display part is lowered, so it has been difficult to obtain a sharp display. Moreover, since the white liquid is sealed in these display media, there is a risk of leakage of the white liquid from the display media when the display media are detached from image display devices and handled in a rough manner like paper.
The twisting ball display is a method for display by rotating spherical particles, with one half-surface painted in white and the other half-surface painted in black, by the function of an electric field, for example, functioning the electric field such that the black surface is on a display surface side in the image part and the white surface is on the display surface side in the non-image part. According to this, since rotation driving of the particles is not generated except by the function of the electric field, the display memory property can be provided. Moreover, in the display medium, since an oil is present only in cavities in the vicinity of the particles but the display medium comprises mostly a solid, the display medium can be formed as a sheet relatively easily. However, according to this method, it is difficult to completely rotate the particles and the contrast is deteriorated for particles without complete rotation, so a sharp display image is hard to form. Furthermore, even in the case the half spherical surfaces painted in white can completely align on the display side, it is difficult to provide a white display like paper, due to light absorption and light scattering in the cavities, and consequently it has been difficult to obtain a sharp display image. Moreover, since the particle size is required to be smaller than a pixel size, a problem is involved in that fine spherical particles painted in colors need to be produced for a high resolution display, so a highly sophisticated production technique is required.
Moreover, recently, as a completely solid-type display medium, several display media with colored particles, such as a powdery toner, sealed between a pair of substrates (for example, the display media disclosed in Japan hardcopy, '99 articles, pp. 249–252, Japan Hardcopy, '99, Fall preliminary articles, pp. 10–13, and Japanese Patent Laid-Open (JP-A) No. 2000-347483, and a display medium disclosed in JP-A No. 2001-33833 have been proposed.
These have a configuration comprising a conductive colored toner (such as a black toner) and an insulating colored particle (such as a white color particle) sealed between a transparent display substrate and a rear substrate facing thereto with a minute gap. An electrode is formed on the display substrate and the rear substrate, and the inner surface of each substrate is coated with a charge transporting material for transporting only charge of one polarity (for example, positive holes).
When a voltage is applied between these substrates, positive holes are injected into only the conductive black toner, and the black toner is charged positively so as to move between the substrates, pushing past the white particles, according to the electric field formed between the substrates. Here, where the black toner is moved to the display substrate side, black display is executed, and where the black toner is moved to the rear substrate side, white display is executed by the white particles. Therefore, by applying a voltage between the substrates for moving the black toner according to the image information of the image to be displayed, black-and-white image display can be carried out.
According to the display medium using these colored particles, since the particles are not moved as long as an electric field is not applied, the display memory property can be provided. Moreover, since the display medium only contains solids, a problem of liquid leakage cannot occur. Furthermore, a high contrast image display can be provided by two kinds of colored particles (for example, the white particles and black particles).
Moreover, as a display medium proposed by the present inventors, one disclosed in Japanese Patent Application No. 2000-165138 can be presented. This has a configuration comprising two kinds of colored particle groups having different colors and charge characteristics sealed between a transparent display substrate and a rear substrate, with the two kinds of colored particle groups charged with opposite polarities. Then, by operating an electric field between the display substrate and the rear substrate, the two kinds of colored particle groups are moved independently to the different substrate sides for executing the display. According to the display medium, by applying a voltage between the substrates according to image information, a high contrast, sharp image display can be executed.
Furthermore, since the display medium has a characteristic of not providing the image display until a certain applied voltage (threshold voltage) is applied, a simple matrix drive can be adopted for driving the particles, and thus a low cost for a driving controller can be achieved.
However, in the above-mentioned display medium disclosed in Japanese Patent Application No. 2000-165138, when a simple matrix drive with linear electrodes provided at the display substrate and the rear substrate is used, since the particles are moved along the spread of the electric field generated between the substrates, the particles cannot be moved perpendicularly with respect to the substrates.
In particular, when a display drive voltage is applied such that a potential difference is generated between the electrodes adjacent to the linear electrodes, since a distance between adjacent electrodes is small compared to the distance between the substrates facing each other, a strong electric field (an edge electric field) is formed between the adjacent electrodes, so the colored particles in the vicinity of the adjacent electrodes are moved and spread out by the edge electric field. For example, according to results of experiments by the present inventors and others, if the distance between the electrodes facing each other is 200 μm, the image edge part is expanded by about 100 μm to 150 μm. When an image with a low resolution of about several tens of dpi (dots per inch), this is in the degree of having image edge parts slightly blurred, and thus does not provide a significant influence visibly. However, when an image with a high resolution exceeding 100 dpi, blurring and distortion of the image are conspicuous and drastically deteriorate the display quality.
Moreover, in the case the display drive voltage is constant, since the number of colored particles moved from the rear substrate side so as to be adhered on the display substrate is substantially fixed, if the particles are moved in a spread manner, then the particle density per unit area in the display part is smaller and the image density is lowered. When an image with a low resolution of several tens of dpi or less, this is again in the degree of having the image edge parts slightly blurred, and does not provide a significant influence visibly. However, when an image of a high resolution exceeding 100 dpi, density deterioration of a dot image, line image, character image, or the like remarkably deteriorates the display quality.
Accordingly, although a good image with high contrast and without conspicuous particle spread can be obtained when a low resolution is used, a problem is involved of thickening of a line image along the linear electrodes formed on the rear substrate, and the line density deterioration becomes remarkable when a higher resolution is used, so as to deteriorate the display quality.
Similarly, when forming a single electrode on the entire surface of the display substrate and forming pixel electrodes corresponding to one pixel per pixel on the rear substrate for adopting an active drive, a good image with a high contrast can be obtained for a display medium of low resolution. However, if the resolution of the display medium is higher, the particles cannot move perpendicularly with respect to a selected pixel electrode, and thus a problem is involved in that blurring and density deterioration of display dots become conspicuous, so as to deteriorate the display quality.